hslee

KIMS Limit on WIMP Cross Section: 

KIMS Limit on WIMP Cross Section Hyun Su Lee Seoul National University (For KIMS Collaboration)

Index: 

Index CsI crystal & background reduction Why CsI? Internal background & reduction Our setup & data taking Detector & DAQ Calibration Analysis Limit on SD & SI Future plan

Why CsI(Tl) Crystal ?: 

Advantage High light yield ~60,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I) Relatively easy to get large mass with an affordable cost Disadvantages Emission spectra does not match with normal bi-alkali PMT => Effectively reduce light yield 137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic Why CsI(Tl) Crystal ? CsI(Tl) NaI(Tl) Photons/MeV ~60,000 ~40,000 Density(g/cm3) 4.53 3.67 Decay Time(ns) ~1050 ~230 Peak emission(nm) 550 415 Hygroscopicity slight strong

Pulse Shape Discrimination: 

Pulse Shape Discrimination Quality Factor

Background of CsI Crystal: 

Background of CsI Crystal 137Cs (artificial) serious background at low energy 134Cs (artificial+133Cs(n,gamma)) 87Rb (natural) Hard to reject  reduction technique in material is known Single Crystal (~10 kg) background @ ~10keV 87Rb 1.07 cpd/1ppb HR ICP-MASS 137Cs 0.35 cpd/1mBq/kg HPGe 134Cs 0.07 cpd/1mBq/kg HPGe Pollucite( raw material for Cs ) contains < 8 mBq/kg of 137Cs 137Cs : 10 mBq/kg 134Cs : 20 mBq/kg 87Rb : 10 ppb 87Rb 137Cs 134Cs keV CPD Geant Simulation

Internal Background Reduction: 

Internal Background Reduction CsI solubility in water is very high. Recrystallization is done at slightly lower temperature from saturation point. 10 ppb powder  ~ 1 ppb (< 1.1cpd) Rb87 Reduction Cs137 Reduction Water is main source of Cs137 It was reduced by using purified water

Background Spectrum of current crystal: 

Background Spectrum of current crystal Cs137 : 7.42 +- 1.50mBq Cs134 : 21.1 +- 1.87 mBq Rb87 : 0.8mBq (0.93ppb) ~ 5.5 cpd (10keV)

Cs134 Rejection (Geant4 simulation): 

Cs134 Rejection (Geant4 simulation) 134Cs : t 1/2 = 2.065 year : Artificial+ 133Cs(n,) b to 134Ba* (Q=2058.7 keV)  prompt  emission Can be rejected easily using gamma tag

How much can we reduce in the next step?: 

How much can we reduce in the next step? Rb87 ~1 cpd Can be reduced by repeat of re-crystallization ~order of 0.1 cpd Cs137 ~3 cpd Can be reduced by ultra pure water ~less than 1 cpd Cs134 ~ 2 cpd Can be reduced by gamma tagging ~ 0.1cpd ~<2cpd Powder selection Pure water Pure water + Recrystallization Ultrapure water + Recrystallization + Crystal array

Shielding for External background reduction: 

Shielding for External background reduction Shielding Pb(15cm), Cu(10cm) LSC(30cm), PE(5cm) YangYang Underground Lab 700 m (minimum depth)

CsI(Tl) Detector: 

CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg) 3” PMT (9269QA) Quartz window, RbCs photo cathode 4~5 Photo-electron/keV DAQ 500MHz Home Made FADC 5 photo-electron within 2μsec trigger condition total 32μsec window CsI(Tl) Detector Pedestal Signal Typical Fe55 Signal

DATA: 

DATA Data for WIMP search 430 kg days for 8x8x30 cm3 crystal of 14 cpd background level(2004 spring) 237 kg days for 8x8x23 cm3 crystal of 5.5 cpd background level(2004 summer) 200 kg days for 8x8x23 cm3 crystal of 5.5 cpd background level(2004-5 winter) MC data using Geant4 simulation Calibration data Neutron data from Neutron calibration facility Reference distribution for recoil events - equivalent to 2000 kg days of underground data Fe55(5.9keV) Calculation of Photo electron Yield Am241(59.5keV) Energy Calibration 137Cs ( 661.657 keV g ) Compton data Reference distribution for γ events equivalent to 1800 kg days of underground data Cut efficiency calculation

Event Selection: 

Event Selection Cs137 BG Cs137 BG Mean Charge of SPE Mean Charge of SPE Biggest SPE Charge Biggest SPE Charge Energy Energy Energy Energy PMT background related events were removed

Efficiency: 

Efficiency Efficiency was calculated by Cs137 low energy calibration data Reference cut efficiency for Cs137, which was calculated by Fe55 calibration data, was included to systematic error Low Energy Spectrum without & with cut Without Cut With Cut

Neutron calibration facility in SNU: 

300 mCi Am/Be source  neutron rate 7 x 105 neutrons /sec  a few 100 neutrons/sec hit 3cmX3cm crystal  Quenching factor of Recoil Energy Take Neutron calibration data PSD check – Quality factor @Energy = 10 keV 137 Cs Compton Neutron Recoil Background data Neutron calibration facility in SNU

Log(Mean Time) of DATA & Gamma & Neutron: 

Log(Mean Time) of DATA & Gamma & Neutron 3~4 keV 4 ~5 keV 11~12 keV 10~11 keV 9~10 keV 8~9 keV 7~8 keV 6~7 keV 5~6 keV

Histogram Fitting Results: 

Histogram Fitting Results Background level WIMP mass 20 GeV/c2 60 GeV/c2 130 GeV/c2 250 GeV/c2

WIMP Proton Cross Section : 

WIMP Proton Cross Section WIMP Nucleus Scattering Spin Independent Spin Dependent ρχ=galatic halo density vE=earth velocity in galatic frame v0=sun velocity in galatic frame

Spin Independent Limit: 

Spin Independent Limit Dark matter density at the solar system rD = 0.3 GeV c-2 cm -3 Use annual average parameters V0 = 220 km s-1, VE = 232 km s-1, VEsc = 650 km s-1

Spin Dependent Limit: 

Spin Dependent Limit WIMP – Proton <Sp>Cs = 0.370 <Sp>I = 0.309 WIMP – Neutron <Sn>Cs = 0.003 <Sn>I = 0.075

Summary: 

Summary We successfully reduce internal background of CsI (Tl) crystal Now we achieve 5.5cpd level at 10keV region We successfully take long term data for WIMP search using 5.5 cpd crystal Neutron & gamma calibration carried out PSD is well working at low energy Successfully extract nuclear recoil (wimp-like) events We can give a competent limit on WIMP for SD and SI using current data

Prospects: 

Prospects Now we start measurement with 3 crystal All crystal was made by using same quality powder We expect same background level ~ 5.5cpd Total ~24kg 6.4kg 8.7kg 8.7kg Future plan More purified powder was produced about 300kg Ultra pure water use Further recrystallization Crystal array Less than 2cpd background level expected 250kg crystal can be installed in current Cu box 1ton can be installed by modification Cu box

Projected limit of KIMS for SI interaction: 

Projected limit of KIMS for SI interaction (2cpd 250kg year)

Projected limit of KIMS for SD interaction: 

Projected limit of KIMS for SD interaction WIMP - Proton WIMP - Neutron